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doc: add more questions to FAQ

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Miroslav Lichvar 2023-07-20 10:59:05 +02:00
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@ -43,6 +43,33 @@ For a more detailed comparison of features and performance, see the
https://chrony.tuxfamily.org/comparison.html[comparison page] on the `chrony`
website.
=== Should I prefer `chrony` over `timesyncd` if I do not need to run a server?
Generally, yes.
`systemd-timesyncd` is a very simple NTP client included in the `systemd`
suite. It lacks almost all features of `chrony` and other advanced client
implementations listed on the
https://chrony.tuxfamily.org/comparison.html[comparison page]. One of its main
limitations is that it cannot poll multiple servers at the same time and detect
servers having incorrect time (falsetickers in the NTP terminology). It should
be used only with trusted reliable servers, ideally in local network.
Using `timesyncd` with `pool.ntp.org` is problematic. The pool is very
robust as a whole, but the individual servers run by volunteers cannot be
relied on. Occasionally, servers drift away or make a step to distant past or
future due to misconfiguration, problematic implementation, and other bugs
(e.g. in firmware of a GPS receiver). The pool monitoring system detects such
servers and quickly removes them from the pool DNS, but clients like
`timesyncd` cannot recover from that. They follow the server as long as it
claims to be synchronised. They need to be restarted in order to get a new
address from the pool DNS.
Note that the complexity of NTP and clock synchronisation is on the client
side. The amount of code in `chrony` specific to NTP server is very small and
it is disabled by default. If it was removed, it would not significantly reduce
the amount of memory or storage needed.
== Configuration issues
=== What is the minimum recommended configuration for an NTP client?
@ -573,6 +600,24 @@ transport for NTP messages (NTP over PTP) to enable hardware timestamping on
hardware which can timestamp PTP packets only. It can be enabled by the
`ptpport` directive.
=== How can I avoid using wrong PHC refclock?
If your system has multiple PHC devices, normally named by `udev` as
_/dev/ptp0_, _/dev/ptp1_, and so on, their order can change randomly across
reboots depending on the order of initialisation of their drivers. If a PHC
refclock is specified by this name, `chronyd` could be using a wrong refclock
after reboot. To prevent that, you can configure `udev` to create a stable
symlink for `chronyd` with a rule like this (e.g. written to
_/etc/udev/rules.d/80-phc.rules_):
----
KERNEL=="ptp[0-9]*", DEVPATH=="/devices/pci0000:00/0000:00:01.2/0000:02:00.0/ptp/*", SYMLINK+="ptp-i350-1"
----
You can get the full _DEVPATH_ of an existing PHC device with the `udevadm
info` command. You will need to execute the `udevadm trigger` command, or
reboot the system, for these changes to take effect.
=== Why are client log records dropped before reaching `clientloglimit`?
The number of dropped client log records reported by the `serverstats` command